Low temperature germinability (LTG) is an essential character in rice varieties adapted to direct seeding. However, slow progress of researches had been inevitablly due to the com-plexity of physiological function of LTG.

In this report, 81 recombinant inbred lines (RILs), derived from Kinmaze (japonica) / DV85 (indica), was used for molecular mapping of QTLs for LTG. The seeds of three plants per RIL and parental line with three replicates were harvested at 35 days after heading and treated at 50C for 10 days in order to break the dormancy. The seeds with a coleoptile longer than 2mm were counted as completion of germination. The normal temperature germinability (NTG) as primary dormancy control was evaluated with the 100 seeds of each plant, which were placed on two sheets of filter paper moistened with distilled water in a petri-dish 9cm in diameter, incubated at 30C for 5 days. The LTG was scored after 10 days at 15C by the same method of NTG except for the temperature. After treated 10d at 15C, the seeds were moved into the incubator of 30C. After another 5 days, the germination rates were measured again, which was taken as the secondary dormancy control.

NTG and the germination rate in second dormacy test of all the RILs were more than 80% (data not shown), indicating that LTG was free from the effect of the primary dormancy and secondary dormancy of the seeds. Frequency distribution of the LTG was shown in Fig. 1. The LTG of the RILs ranged from 0% to 99%, which displayed the transgressive distribution over the better LTG parent type (Kinmaze).

QTLMapper Version 1.0 was applied to investigate the association between phenotype and marker genotype (Wang et al. 1999). LOD score of 3.0 was used as criteria to indicate the putative QTLs. Four putative QTLs, qLTG-2, qLTG-7, qLTG-11 and qLTG-12, were detected on chromosome 2, 7, 11 and 12, respectively (Fig. 2, Tabel 1). DV85 alleles increased the LTG at the regions of qLTG-2 and qLTG-11, while Kinmaze alleles increased it at the other two regions. Among these QTLs, qLTG-2, qLTG-7 and qLTG-12 were firstly detected, and qLTG-11 was closed to that previously reported by Wan et al. (1999) and Miura et al. (2001). Meanwhile, two pairs of epistatic loci for LTG were detected (Fig. 3, Tabel 2), which accounted for 9.7% and 8.3% of the total variation, respectively. Both the additive and epistatic effects of qLTG-7 were significant. Moreover, this loci was linked to a dormancy loci reported previously by Wan et al. (1997) and Lin et al. (1998). These results should be very useful for

further study on gene map-based cloning and gene function analysis of LTG.

Acknowledgement

We thank Drs. H. YASUI and A. YOSHIMURA, Graduate School of Agriculture, Kyusyu University, Japan, for kind gift of the materials

References

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